(1) Field of the Invention
The present invention relates to a squelch detecting circuit in a radio receiver and, in particular, to the squelch detecting circuit which does not interfere measurement of transmitting properties of transmitters and receivers.
(2) Description of the Prior Art
A squelch system has been used in a radio receiver in, for example, a VHF band, a UHF band, or a microwave band, in order to block the output of the radio receiver automatically when a carrier received is reduced to or below a predetermined threshold level.
The squelch system comprises a squelch detecting circuit connected to a front end portion of the receiver. The squelch detecting circuit comprises a detector which detects an envelope of a predetermined carrier frequency in a signal inputted thereto. The detected envelope signal is applied to a voltage comparator which, in turn, produces a squelch alarm signal when the detected envelope signal is equal to, or lower than, a predetermined squelch level corresponding to the threshold level. The squelch alarm signal is applied to a squelch gate, which in turn, mutes the output from the receiver.
Generally speaking, the front end portion of the receiver has a wide frequency band sufficient to protect a desired carrier frequency signal from distortion. Accordingly, the signal applied to the squelch detecting circuit comprises not only the desired carrier frequency signal component but also adjacent frequency signal components. Therefore, no squelch alarm signal is produced from the squelch detecting circuit due to existence of the adjacent frequency signal components even when the desired carrier frequency signal is reduced to or below the threshold level.
In order to resolve such an erroneous operation of the squelch detecting circuit, it is known in the prior art to use a narrow bandpass filter in the squelch detecting circuit. The narrow bandpass filter is for eliminating all but those frequencies close around the desired carrier frequency. A filtered signal passing through the narrow bandpass filter is applied to the detector.
In use of the narrow bandpass filter, the operation of the squelch detecting circuit is not interfered by existence of the adjacent frequency signal components but the squelch alarm signal is reliably produced in response to only the desired carrier frequency signal reduced to or below the threshold level.
On the other hand, a test is sometimes performed for measuring transmission characteristics, for example, the amplitude-frequency characteristic, the delay response, or others of a transmitter and a receiver. In such a test, the carrier is frequency swept repeatedly by a predetermined low frequency, for example, a frequency (50 Hz or 60 Hz) of a commercial electric power source across a proper or assigned frequency of the carrier.
In the test using the carrier frequency sweeping, the squelch detecting circuit having the narrow bandpass filter produces the squelch alarm signal each time when the carrier frequency is swept out of the passband of the narrow bandpass filter, so that the transmission characteristics cannot correctly measured.
Japanese Utility Model (U.M.) Publication No. 13,643/83 proposes a resolution of the problem. The Japanese U.M. discloses to use a time constant circuit so that the detected envelope signal is applied to the voltage comparator through the time constant circuit. In the arrangement, the detected envelope signal is integrated at the time constant, the integrated voltage reduces at the same time constant when the detected envelope signal becomes zero level. Accordingly, the input voltage level of the voltage comparator is generally maintained above the squelch level, even if the carrier frequency is swept out of the passband of the narrow bandpass filter. Thus, start of the squelch alarm signal is delayed so that no squelch alarm signal is produced during the test.
Reviewing the purpose of the squelch system, the narrow bandpass filter is desired to have a narrower passband and the squelch signal is desired to be stopped immediately when the desired carrier signal is recovered above the threshold level.
However, the narrower bandpass filter cannot be used in the squelch detecting circuit using the time constant circuit because the output signal level of the time constant circuit becomes below the squelch level during a time when the carrier frequency is swept out of the passband of the narrower bandpass filter so that the squelch alarm signal is produced.
Further, once the squelch alarm signal is produced from the squelch detecting circuit using the time constant circuit, the squelch alarm signal is not stopped immediately when the desired carrier signal is recovered above the threshold level. This is because recovery of the input signal level of the voltage comparator is delayed by the time constant circuit.